[Qemu-devel] Re: NBD server for QEMU images

[Salvador Fandino <sfandino@yahoo.com>]

Martin Guy wrote:
>> The patch available from http://qemu-forum.ipi.fi/viewtopic.php?t=2718
>> adds a new utility, qemu-nbds, that implements a NBD server
> 
> I have been using nbd volumes mounted from inside qemu for filestore
> and for swap, both read-write, served from files and from partitions,
> with the unmodified standard nbd-server (debian testing version) for
> intensive work and it has been faster and more reliable than NFS (not
> that that's saying much).
> 
> The only thing that doesn't work is the -swap option, which just
> hangs, but that proves not to be necessary when swapping onto nbd host
> volume from qemu-land, even when stress-testing it.
> 
> What problem is solved by a specially modified nbd server?

It serves disk images in any format QEMU can handle, for instance, qcow
images.

It's mostly intended to be used for accessing the files inside QEMU disk
images locally, without having to launch a virtual machine and accessing
then from there.

For instance, if you use QEMU to run Windows, and at some point you need
to get some file from your emulated windows disk, you can do it as follows:

 $ qemu-nbds windows.qcow -p 8001 -o 32256 &

 # modprobe nbd
 # nbd-client localhost 8001 /dev/nbd0
 # mount -o ro /dev/nbd0 /mnt/windows

 $ cp /mnt/windows/FOO.txt ~/


Cheers,

 - Salva



Cheers,

 - Salvador.